The present invention relates to modems, in general, and more particularly to a modem for selecting a carrier frequency and a baud rate from a predetermined plurality of carrier frequencies and baud rates to communicate with another modem over a communication media of a data communications network in a full duplex mode the selection being based on estimated characteristics of the communication media, and a method of operating a modem by combining the processes of estimating channel characteristics and estimating range in a common startup procedure comprising a plurality of successive time segments.
In a data communication network, digital data among other data, may be communicated at a data bit rate from one modem to another modem through a communication media, which may be a leased line of the network or a dial up connection of a general switched telephone network (GSTN), for example. Generally, modems operate at a fixed carrier frequency and a fixed modulation or baud rate and attempt to optimize the data exchange bit rate based on the conditions of the communication media over which they are communicating. In order to accomplish an optimum data bit rate, contemporary modems utilize a startup learning procedure before commencing communication during which the modems perform certain predefined start up procedures which may include a line probing sequence, for example, to establish the media characteristics over which communication will take place. The current state of the art CCITT standard for two wire full duplex modems is V.32 and V.32bis. An example of a modem employing the V.32 standard today includes the Codex Model 2264. An example of a state of the art modem using line probing is the Codex Modem Model 3680.
In addition, two wire modems for operating in a full duplex mode generally employ an echo canceller to cancel from the received signal any near end and far end echoes resulting from its concurrent signal transmissions. These modems include a ranging sequence as part of the startup procedure to determine the round trip signal delay time over the media to and from a remote modem which is used by the echo canceller thereof. The ranging task is performed separate and distinct from any line probing tasks. Further, in two wire, full duplex transmission systems, there are system nonlinearities which affect not only the signal transmission, but also both of the near end and far end echoes resulting therefrom. Conventional line probing training sequences do not measure the nonlinearities of the echo signals and, for this reason, cannot provide adequate estimates for echo cancellation purposes.
Still further, if during data communication between two modems, a malfunction, like loss of synchronization, is detected, the modems presently on the market resort to breaking communications and repeating the entire startup procedure, including both line probing and ranging tasks, which is a very lengthy and cumbersome retraining process. Moreover, only one of the communicating modems is generally designated to initiate this retraining process upon malfunction which adds further complications.
The present invention offers aspects intended to alleviate the aforementioned drawbacks of the current modems. These aspects will be better understood from a description of the preferred embodiment found hereinbelow taken together with the accompanying drawings.